Are you saying MapleStory lags when you play it..?
How does it lag..?
If it "lags" when you try to pick something up, or shoot, or go through a portal, or the monsters pause in place (they keep animating, but don't move around), that's "Network Lag". You fix that by finding out what the problem is with your Internet connection.
If it "lags" when YOU move, or try to jump, or the screen "jerks" when you move, that's "Computer Lag". You fix that by not running YouTube in the background or iTunes or music players or other games and lots of other things while you're playing. You can also help that kind of lag by replacing your old video card with a more capable one, or as CrazeeFun suggested, by running "GameBooster" to stop unnecessary Windows processes while you play.
Try GameBooster for sure- it's free and works pretty well with older computers. Newer, faster computers probably won't see any improvement, but it's an easy thing to try anyway. Google the name for the legit download link.
try to delete useless stuff on ur computer to gain disk space, buying more ram, lowering the graphics in game, full screen runs faster if u play in windowed, there are utilities that boost power and speed to a certain program (ex. a game) yea.